Oilseed crushing, the production of vegetable oil and animal feed from oilseeds, nearly always entails a solvent extraction step in which particulate, oilseed material is contacted with an organic solvent such as but not limited to hexane and its isomers to separate the liquid fraction from the solid fraction. The resultant liquid fraction is known in the trade as “miscella”, a solution of vegetable oil in the organic solvent, and the resultant solid fraction is known in the trade as “spent material”, the extraction residue soaked with organic solvent. After evaporating the solvent from the miscella, the crude vegetable oil is further processed to produce edible oil products. After evaporating the organic solvent present in the spent material, an oilseed meal is produced that is primarily used as a high protein animal feed ingredient.
The spent material from the solvent extraction process is generally soaked with twenty-five to thirty-five percent (25-35%) solvent by weight. The downstream process of evaporating the solvent from said spent material is known in the trade as “desolventizing”, whose primary purpose is to remove the organic solvent to as low a residual solvent content as possible for both feed safety and environmental purposes.
Most oilseeds contain anti-nutritional factors (ANF) that prevent key amino acids in oilseed meals from being digested by mono-gastric animals such as poultry and swine. Said ANF generally consist of heat labile peptides such as the trypsin inhibitors present in soybeans. Accordingly, heating the oilseed meal to a sufficiently high temperature with adequate moisture present for a sufficiently long period inactivates these ANF by denaturing said peptides. The term used in the trade for this process is “toasting”.
Toasting the meal simultaneously denatures the protein present in the meal. Denaturing the protein lowers the availability of the protein to be digested by mono-gastric animals such as poultry and swine, which therefore lowers the feed value of the meal in these markets. The degree of protein digestibility is often characterized in the trade by the protein digestibility index (PDI) expressed in percent. Higher PDI means higher availability of the protein to be digested. The majority of soybean meal in the trade today has a PDI in the range of twenty to thirty-five percent (20-35%).
The prior art, as exemplified by U.S. Pat. No. 5,992,050, employs equipment whereby spent material is treated in an apparatus that is known in the trade as a “desolventizer-toaster” (DT). The solvent content of the spent material is generally twenty-five to thirty-five percent (25-35%) by weight, with a moisture content of ten to twelve percent (10-12%) by weight and a temperature of 50-60° C.
The spent material continuously enters the DT vessel and drops onto a tray inside, known in the trade as a “pre-desolventizing” (PD) tray, which is an indirect steam-heated horizontal disc with a surface temperature typically 135-155° C. A shallow layer of spent material is stirred above the surface of the PD tray by stirring blades extending from a rotating centrally located shaft. Upon contacting the PD tray surface, the temperature of the spent material is increased to the boiling range of the solvent, typically 60-70° C., where the organic solvent begins to evaporate. A typical DT has two to four PD trays stacked vertically in series, where approximately ten to twenty percent (10-20%) of the initial solvent is evaporated, while the temperature remains 50-60° C. and the moisture remains relatively constant at ten to twelve percent (10-12%).
After the spent material exits the PD trays, the spent material continuously enters the next chamber of the DT vessel. The spent material drops onto a tray inside, known in the trade as a “counter-current” (CC) tray, which is an indirect steam-heated horizontal disc with a surface temperature typically 135-155° C. that has special apertures therein that allow ascending superheated water vapor to rise through. A deep layer of spent material is stirred above the surface of the CC tray by stirring blades extending from a rotating centrally located shaft. Soon after falling to the material surface of spent material supported above the CC tray, rising superheated water vapor from below condenses into the spent material layer providing latent heat to evaporate the majority of the remaining solvent from the spent material. When the spent material exits this first CC tray it will typically contain less than one percent residual solvent by weight with a moisture content of eighteen to twenty-two percent (18-22%) by weight and a temperature of 95-100° C. These moisture and temperature conditions allow for destruction of ANF and reduction of protein digestibility in the spent material.
A typical DT has two to four CC trays stacked vertically in series. After the first CC tray, the temperature and moisture remain relatively constant. The primary purpose of the remaining CC trays is to allow time for further stripping of the remaining traces of residual solvent by rising superheated water vapor and time for adequate destruction of the ANF. Reduction of the protein digestibility of the spent material continues on the remaining CC trays.
After the spent material exits the final CC tray, the spent material continuously enters the next chamber of the DT vessel. The spent material drops onto a tray inside, known in the trade as a “sparge” (SP) tray, which is a hollow horizontal disc with perforated upper surface for even distribution of superheated water vapor into the DT. A deep layer of spent material is stirred above the surface of the SP tray by stirring blades extending from a rotating centrally located shaft. The evenly distributed superheated water vapor rises through the spent material to achieve efficient removal of the remaining residual solvent to acceptable levels. Final destruction of ANF and reduction of protein digestibility also occur on the SP tray. When the desolventized, toasted meal exits the DT vessel it will typically contain less than 0.05% solvent by weight with a moisture content of eighteen to twenty-two percent (18-22%) by weight and a temperature of 105 to 110° C. The ANF of the desolventized, toasted meal as measured by urease delta pH is typically less than 0.15 and the protein digestibility as measured by PDI is typically 20-30%.
The solvent evaporated from the spent material along with an equilibrium quantity of water vapor typically exits the upper roof of the DT vessel. This vapor stream is generally 67-75° C. temperature and is composed of 90-95% solvent vapor in equilibrium with 5-10% of water vapor. This vapor stream often contains traces of meal dust and therefore passes through a vapor scrubbing apparatus before passing on to heat recovery.
In summary, it is known in the trade that the DT operates at or near atmospheric pressure and therefore the temperature of the spent material reaches a temperature just above the boiling point of water, 105-110° C. It has been found that with a typical DT apparatus and process that the amount of superheated water vapor condensation required to evaporate the remaining solvent in the spent material after the PD trays typically increases the moisture of the spent material to 20-22% by weight. It also been found that to achieve a residual solvent content in the desolventized, toasted meal at the exit of the DT of less than 500 parts per million (ppm), and preferably less than 250 ppm solvent by weight, that the spent material must remain in the CC and SP trays of the DT collectively for 20 minutes or greater with a minimum rising superheated water vapor flux rate of 300 kg/hr water vapor per m2 tray surface area. It is further known that the combination of 20-30 minutes residence time at 105-110° C. and 20-22% moisture produces a meal with a protein digestibility as measured by PDI of 20-30%. This represents the present state-of-the-art apparatus, process and results in the trade.
Accordingly, there is a need in the art to provide a DT process and apparatus wherein the resultant desolventized and toasted spent material meets the industry requirements for residual solvent content and destruction of ANF, while simultaneously providing a meal product with significantly high protein digestibility as measured by PDI.